The goal of this proposal is to understand the mechanism by which eukaryotic cells target new membrane growth and secretion to specific sites on the cell surface and how this is coordinated with changes in cell polarity during the cell cycle. In this proposal we will examine the specific role of Lgl/Sro7 and Rho/Cdc42 protein families in this process. The Lgl/Sro7 family of proteins, first.identified as a tumor suppressor in Drosophila, is found in all eukaryotic cells and is likely to have a highly conserved function in regulation of polarity and exocytosis. We have characterized homologs of this family in both yeast and mammalian epithelial cells where they are found in physical association with a specific set of SNARE proteins. We will test the hypothesis that Lgl family members have a conserved structure and function in polarized exocytosis and may act as effectors of Rab GTPases in mediating SNARE-dependent vesicle fusion during polarized growth. Genetic analyses performed in our lab have led to the identification of a direct role for Rho/Cdc42 GTPases in exocytosis. In particular we have found two Rho GTPases, Rho3 and Cdc42, that function in a pathway which involves spatial regulation of exocytosis through direct activation of a multisubunit complex known as the Exocyst. In this proposal we will take a comprehensive approach to identify the molecular mechanism by which Sro7, Rho3, and Cdc42 function in the regulation of polarized exocytosis. Ultimately understanding the molecular details of these processes may allow the development of new approaches and novel therapeutics to combating cancer, or type II diabetes, as well as other diseases in which regulation of cell surface trafficking is central to the etiology of the disease.